ALBERT

Description: Trichodesmium spp. is a colonial diazotrophic cyanobacterium found in the oligotrophic (sub)tropical oceans, where dissolved inorganic phosphorus (DIP) can be depleted. To cope with low P concentrations, P can be scavenged from the dissolved organic P (DOP) pool. This requires the deployment of multiple enzymes activated by trace metals, potentially enhancing metal requirements under stronger P limitations. To test this, we grew Trichodesmium under trace-metal-controlled conditions, where P was supplied as either DIP or DOP (methylphosphonic acid). Mean steady-state biomass under the DOP treatment was only 40% of that grown under equivalent DIP supply, carbon normalized alkaline phosphorus activity was elevated 4-fold, and the zinc (Zn)–carbon ratio was elevated 3.5-fold. Our finding matches the known, dominant Zn requirement across a diversity of enzymes involved in P stress responses and supports an important interaction in the oceanic cycles of these two nutrients.

Description: Fast repetition rate fluorometry (FRRf) based on active chlorophyll fluorescence is a powerful, noninvasive tool for studying phytoplankton physiological status at high spatial and temporal resolution. The South China Sea (SCS) is one of the largest tropical–subtropical marginal seas in the world, which plays an important role in modulating regional carbon budget and climate. In this study, underway in situ FRRf measurements were carried out throughout the outer continental shelf of the northern SCS, the basin of the northern SCS, the cyclonic eddy influenced domain in the western SCS, and the basin of the southeastern SCS. Pronounced diurnal variability of FRRf-derived parameters were observed, characterized by a large midday depression and slight nocturnal depression of the maximum quantum yield of photosystem II ( F v /F m ) and a slight increase in the functional absorption cross-section of photosystem II photochemistry (σ PSII ) at noon. F v /F m at the surface was typically as low as 0.1–0.3 and exhibited higher values (~0.4) where internal waves occurred. The cyclonic eddy increased F v /F m slightly, implying that it had a limited impact on surface phytoplankton photophysiology. With proper interpretation, FRRf has been a powerful tool to assess the physiological status of phytoplankton in the sea and to correlate that to ocean dynamics in an unprecedented fine scale.

Description: Fast repetition rate fluorometry (FRRf) allows for rapid non-destructive assessment of phytoplankton photophysiology in situ yet has rarely been applied to Trichodesmium. This gap reflects long-standing concerns that Trichodesmium (and other cyanobacteria) contain pigments that are less effective at absorbing blue light which is often used as the sole excitation source in FRR fluorometers—potentially leading to underestimation of key fluorescence parameters. In this study, we use a multi-excitation FRR fluorometer (equipped with blue, green, and orange LEDs) to investigate photophysiological variability in Trichodesmium assemblages from two sites. Using a multi-LED measurement protocol (447+519+634 nm combined), we assessed maximum photochemical efficiency (Fv/Fm), functional absorption cross section of PSII (σPSII), and electron transport rates (ETRs) for Trichodesmium assemblages in both the Northwest Pacific (NWP) and North Indian Ocean in the vicinity of Sri Lanka (NIO-SL). Evaluating fluorometer performance, we showed that use of a multi-LED measuring protocol yields a significant increase of Fv/Fm for Trichodesmium compared to blue-only excitation. We found distinct photophysiological differences for Trichodesmium at both locations with higher average Fv/Fm as well as lower σPSII and non-photochemical quenching (NPQNSV) observed in the NWP compared to the NIO-SL (Kruskal–Wallis t-test df = 1, p 〈 0.05). Fluorescence light response curves (FLCs) further revealed differences in ETR response with a lower initial slope (αETR) and higher maximum electron turnover rate ((Formula presented.)) observed for Trichodesmium in the NWP compared to the NIO-SL, translating to a higher averaged light saturation EK (= (Formula presented.) /αETR) for cells at this location. Spatial variations in physiological parameters were both observed between and within regions, likely linked to nutrient supply and physiological stress. Finally, we applied an algorithm to estimate primary productivity of Trichodesmium using FRRf-derived fluorescence parameters, yielding an estimated carbon-fixation rate ranging from 7.8 to 21.1 mgC mg Chl-a–1 h–1 across this dataset. Overall, our findings demonstrate that capacity of multi-excitation FRRf to advance the application of Chl-a fluorescence techniques in phytoplankton assemblages dominated by cyanobacteria and reveals novel insight into environmental regulation of photoacclimation in natural Trichodesmium populations